In the lithography employed for semiconductor production, formation of a finer pattern is necessary with an increase in integration. In the formation of a fine pattern, a light source of short wavelength is essential. Currently, a lithography using a KrF excimer laser as the light source is becoming the main stream of the semiconductor lithography, and a lithography using an ArF excimer laser is being put into practical use. Further, lithographies using various radiations of short wavelength, such as F2 excimer laser, EUV, X-ray, electron beam and the like are being developed.
With respect to the photoresists used in the semiconductor lithography, the chemical amplification type resist developed by Ito et al. of IBM is essential currently. In this chemical amplification type, its acid-labile protecting group can give rise to an acid catalyst-induced deprotection reaction, whereby the resist can have high sensitivity.
As specific examples of the resist polymer containing such an acid-labile protecting group, there are known, in the KrF lithography, a copolymer containing a recurring unit derived from hydroxystyrene and a recurring unit derived from an acid-labile alkoxystyrene, a copolymer containing a recurring unit derived from hydroxystyrene and a recurring unit derived from an acid-labile alkyl(meth)acrylate, and a polymer wherein part of the recurring unit derived from hydroxystyrene has been protected with acetal; and in the ArF lithography, a copolymer containing a recurring unit derived from a lactone structure-containing (meth)acrylate and a recurring unit derived from an acid-labile alkyl(meth)acrylate.
All of these copolymers are chemical amplification type resists having an acid-labile protecting group which is eliminated by an acid. In a situation that an even finer resist pattern is required, it has become difficult to obtain sufficient resist properties with such a protecting group alone.
Hence, there have been investigated resist resins wherein a copolymer containing a recurring unit having an acid-labile protecting group has an acid-labile, crosslinked portion introduced into its side chain (see, for example, Patent Literatures 1 to 3 shown later).
In these resist resins, the crosslink is severed by an acid catalyst and thereby the dissolution contrast between light-irradiated area and the light-non-irradiated area is heightened. In production of the above resist resin, a bi-functional monomer such as diacrylate or the like is used and a crosslinking reaction also takes place at the side chain of the high-molecular main chain formed; therefore, there have been the following problems. That is, the resulting polymer has an extremely large distribution of molecular weight and resultantly is low in solubility; moreover, a polymer of ultra-high molecular weight tends to be formed, accordingly, there remains, even after decomposition by an acid, a high-molecular component sparingly soluble in an alkaline developing solution, and this remaining high-molecular component produces defects in formation of fine pattern.
Also in the case (i.e. Patent Literature 1) of using, as a resist, a crosslinked polymer having, in the high-molecular side chain, a crosslinked portion having an acid-labile acetal skeleton, there have been a tendency of poor storage stability owing to the very high sensitivity to acid and further the above-mentioned tendency of low solubility and remaining (undissolved) high-molecular component.
Patent Literature 1: JP-A-2001-98034
Patent Literature 2: JP-A-2000-214587
Patent Literature 3: JP-A-2001-106737